Oil and gas companies spend large sums of money in their attempts to find hydrocarbon deposits. They drill exploration wells in their most promising prospects and use these exploration wells not only to determine whether hydrocarbons are present but also to determine the properties of those hydrocarbons, which are present.
For deep offshore fields, before any hydrocarbons can be produced, it is first necessary to spend several years building very expensive platforms with proper oil and gas handling facilities. The design specifications and cost of materials used in these facilities are strongly dependent on the properties of the hydrocarbons, such as gas to oil ratio, viscosity, bubble point pressure, asphaltene precipitation pressure, and so on. The exploration well itself is generally plugged and abandoned not long after it is drilled. However, the information that it provides is often used throughout the life of the oil or gas field.
To determine hydrocarbon properties, oil and gas companies often withdraw some hydrocarbons from the exploration well. Wireline formation testers, such as the Baker Atlas Reservoir Characterization Instrument (RCI) can be lowered into the well for this purpose.
Initially, fluids that are withdrawn may be highly contaminated by filtrates of the fluids (“muds”) that were used during drilling. To obtain samples that are sufficiently clean (usually <10% contamination) so that the sample will provide meaningful lab data concerning the formation, formation fluids are generally pumped from the wellbore for 30–90 minutes, while clean up is being monitored in real time. Then, these withdrawn fluids can be collected downhole in tanks for subsequent analysis in a laboratory at the surface.
Alternatively, for some properties, samples can be analyzed downhole in real time. The present invention relates both to monitoring sample clean up and to performing downhole analysis of samples at reservoir conditions of temperature and pressure.
A downhole environment is a difficult one in which to operate a sensor. Measuring instruments in the downhole environment must operate under extreme conditions and limited space within a tool's pressure housing, including elevated temperatures, vibration, and shock.
U.S. Pat. No. 5,167,149 by Mullins et al. and U.S. Pat. No. 5,201,220 by Mullins et al., are both entitled Apparatus and Method for Detecting the Presence of Gas in a Borehole Flow Stream. The Mullins apparatus of that invention comprises a downhole 8-channel critical angle (and Brewster angle) refractometer to distinguish oil from gas and to estimate the percentage of gas in a fluid.
The traditional method of measuring the index of refraction of a dark fluid (such as a crude oil) is the critical angle refractometer. A diverging beam of light travels through a transparent solid (e.g., glass) and strikes the interface between this transparent solid and some fluid to be measured, which is in contact with the transparent solid. The reflected diverging beam is dimmer at those angles, which are close to a normal to the interface. At such angles, some of the light is transmitted (refracted) into the fluid.
The reflected diverging beam is much brighter at glancing angles. Starting at the Brewster angle, any incident p-polarized light suffers no reflection loss. Starting at the critical angle, all light, regardless of polarization, suffers no reflection loss but is 100% reflected from the interface so that no light is transmitted into the fluid.
The critical angle can be calculated from Snell's Law, n0 sin θ0=n1 sin θ1, for light refracted as it travels from medium n0 to medium n1. The maximum possible refracted angle (as measured from the normal to the interface) is 90° so by substituting θ1=90° into Snells's Law we can calculate the critical angle, θc=arcsin (n1/n0).
At the critical angle, we see a large change in reflected intensity (a bright/dark demarcation), which can be located using a single moveable detector or an array of stationary photodetectors. A single moveable detector would add substantial mechanical complications to a downhole design.
Laboratory instruments often use an array of 1024 or more stationary photodetectors to detect the critical angle. However, mimicking the lab design downhole would be difficult because multiplexers built into photodetector arrays generally do not work above about 95 C. Even with separate high-temperature multiplexers, multiplexing so many very weak signals at the elevated temperatures encountered downhole would be problematic as they would probably have to be stacked to reduce noise. Therefore, downhole, only a few fixed photodetector elements (e.g., 8) are likely to be used for a critical angle refractometer. Of course, with an 8-channel refractometer, as described in U.S. Pat. Nos. 5,167,149 and 5,201,220 mentioned earlier, the refractive index is measured only in 8 steps rather than as a continuum.
Because such a device only measures refractive index in eight coarse steps, it would be difficult for an operator of this device to monitor sample clean up. Sample clean up refers to the transition from filtrate-contaminated fluid to nearly pure formation fluid while pumping fluid from selected depths in the wellbore.
Accurate sample clean up monitoring cannot be provided by processing a course refractive index reading. Thus, there is a need for a method and apparatus, which can measure refractive index along a continuum so that an operator can accurately monitor the refractive index of a formation sample.